A known digital mobile radio-telephone receiver employs a tunable synthesizer for tuning the receiver to a selected channel, a variable gain amplifier ("VGA") for amplifying a radio-frequency signal received over the selected channel (or an intermediate-frequency signal derived therefrom), a demodulator for generating baseband signals from the amplifier output, an analog-to-digital converter ("ADC") for generating digital versions of the baseband signals, and a decoder for recovery of data from the digital baseband signals.
The recovered data can represent either message information, e.g., voice or data information, or control information. Voice-type message information is presented typically to a vocoder (i.e., a voice decoder) for recovery of a voice signal. Control information is provided typically to a central processing unit in the radio-telephone.
The radio-telephone also has an automatic gain control circuit ("AGC"). The AGC forms a closed feedback loop for generating an AGC control signal from the digital baseband signal, and applying the AGC signal to a control input of the VGA to adjust the amplifier gain. A programmable gain amplifier is a version of a VGA, in which the amplifier gain is restricted to discrete steps, and the AGC control signal selects the desired gain.
The AGC signal attempts to maintain the amplitude of the envelope of the digital baseband signals constant, and is therefore indicative of the received signal strength. The VGA gain is adjusted whenever the envelope levels rise above or drop below selected thresholds, thus maintaining the VGA output within a range of amplitudes that are adequate for processing purposes in the downstream components described above.
Adjusting the amplification gain is necessary because the received signal typically varies in strength (i.e., field strength or energy) over time. The strength of received signals varies in response to propogation conditions that vary, e.g., signal path variations resulting from movement of an automobile carrying the radio-telephone.
Propagation conditions that adversely affect signal strength include (a) physical obstructions in the propagation paths of the signals, (b) constructive and destructive interference of multiple signals caused by signal reflections from buildings and other objects, and (c) varying distances between mobile units and base stations.
As mentioned above, the AGC signal, in addition to controlling the VGA gain, is used in generating values indicative of the strengths of the received signals, i.e., received-signal-strength-indicating ("RSSI") values.
For purposes of generating an RSSI value, the above-described AGC feedback loop is closed and allowed to settle, i.e., to reach equilibrium, while the AGC signal controls the VGA gain so as to maintain the baseband signal envelope within the limits of a pre-selected range (e.g., 0 dB+/-0.5 dB). The feedback signal value occurring after a preselected length of time, which is long enough to assure that equilibrium has been reached, is used as a pointer into a calibrated look-up table stored on board the mobile unit to identify the corresponding RSSI value stored therein. This value is applied to the mobile radio-telephone transmitter for up-loading to the current base station.
RSSI values are used in cellular telephone systems, for instance, in a process called hand-off. As a mobile radio-telephone moves within the service area of the cellular system, communication control is handed-off between base stations in the system in order to maintain adequate signal strength for good-quality transmission and reception. In other words, hand-off is the passing of radio-telephone service during a telephone call from a current base station, which is acting as the intermediary for the call, to another base station in a geographically adjacent cell.
In mobile-assisted hand-off ("MAHO"), for instance, the mobile radio-telephone involved in the call participates in hand-off decisions. The mobile radio-telephone receiver periodically tunes to the frequencies of specified control channels of base stations of the geographically adjacent cells, and, for each channel, measures the strength of received control signals, i.e., generates RSSI values for those signals, and transfers or up-loads the RSSI values for the control channels to a central controller for the cellular telephone system via the current base station. The system controller uses the RSSI values in determining when a hand-off to a base station of an adjacent cell is required, and to which cell the mobile radio-telephone should be given. These decisions are sometimes called "locating."
In known cellular systems employing MAHO and designed for digital radio-telephones, the length of time available for RSSI data collection is short. In such systems, in order for each base station to handle calls to or from a number of (e.g., three) mobile radio-telephones at once, the base station time multiplexes communication with each. This means that the base station communicates periodically, one after the other, with each mobile radio-telephone.
Specifically, each of the three mobile radio-telephones operates serially in three time slots (called RECEIVE, IDLE, and TRANSMIT). The duration of each time slot is prescribed by international standards. During the RECEIVE time slot, which lasts, for instance, about 6.7 msec, the mobile radio-telephone receiver demodulates the communication signal received over an active subscriber channel.
During the IDLE and TRANSMIT intervals, the mobile radio-telephone receiver performs RSSI data collection, i.e., tunes to the frequencies of control channels of adjacent cells, allows its AGC feedback loop to settle, generates RSSI values, and re-tunes to its subscriber channel for ordinary communication during the RECEIVE time slot. This all must take place in a short period of time, for instance, about 13.4 msec, a requirement that is difficult to meet.
This requirement is especially difficult to meet when the received signal suffers the effects of rapid fading, common in the cellular environment. Furthermore, the more time used in generating the RSSI values, the less is available for tuning of the synthesizer among control and subscriber channels within the allowed period of time.
Thus, the time constraints on RSSI data collection impose designs constraints on the AGC circuit and the frequency synthesizer of the receiver. These design constraints increase the cost of the design and implementation of these receiver components.